The British government failed to protect a state scientist found stabbed to death after his research helped connect the Kremlin to a high-profile assassination on British soil, a member of the House of Lords has claimed in a letter to the prime minister.

Lord Rooker, a Labour peer, wrote to Theresa May to raise concerns about 14 suspected Russian assassinations in Britain exposed by BuzzFeed News last year – and said the British government had failed in its duty of care towards state scientist Dr Matthew Puncher, who was found dead at home in Oxfordshire in 2016.

Puncher’s body was found riddled with stab wounds weeks after his nuclear research helped a judge determine that the KGB defector Alexander Litvinenko had been assassinated in London by Russia’s secret service. The police and coroner declared it a suicide – concluding that Puncher had somehow managed to stab and slash himself repeatedly with two separate knives before succumbing to his wounds. But BuzzFeed News revealed that US intelligence agencies had passed MI6 evidence connecting Puncher’s death – and 13 others – to Russian state or mafia assassins, yet the police had treated every case as non-suspicious.

Lord Rooker tabled a written parliamentary question to the government on Tuesday asking when the prime minister intended to respond to a letter raising questions about the 14 deaths that he had sent in November. The letter, obtained by BuzzFeed News, asked why the government had repeatedly sent Puncher to Russia on sensitive nuclear research assignments directly before and after a public inquiry used his research to connect the Kremlin to Litvinenko’s killing in 2016. “It was known how explosive the issue was between the UK and Russia, so why was Dr Puncher not withdrawn?” he asked, noting that the prime minister had made public her commitment to “the population being kept safe” and yet “it appears the Government failed in respect of Dr Puncher”. The peer told BuzzFeed News his letter had been acknowledged but he is still yet to receive a response.

How BuzzFeed News broke the story revealing Matthew Puncher's death as a suspected assassination in June 2017

Lord Rooker’s letter also raised questions over why the 14 deaths exposed by BuzzFeed News have been treated as non-suspicious by the British police. He used a speech in the chamber of the House of Lords last June to call for all 14 cases to be fully investigated.

The peer has joined a growing chorus of politicians on both sides of the Atlantic calling for action in response to a spate of suspected assassinations on British soil that the government has, so far, ignored. Ben Bradshaw, the former Labour cabinet minister, demanded a proper investigation into “unexplained Russia-related deaths in Britain” in a speech in the House of Commons last November in the wake of BuzzFeed News’ investigation, and the matter has also been raised by the former Tory minister John Whittingdale.

The Commons intelligence and security committee has convened an investigation into Russian interference in the UK and its members are understood to be examining the evidence published by BuzzFeed News.

An inquest into one of the 14 deaths – that of Alexander Perepilichnyy, a financier who dropped dead in Surrey after blowing the whistle on massive Russian money laundering scheme – was upended in June after BuzzFeed News exposed explosive intelligence connecting his death directly to the Kremlin. It has now been delayed till the spring while the coroner seeks fresh evidence from the government and the police in the wake of the fresh revelations.

In the US, a report by Democratic members of the Senate’s foreign relations committee this week warned Western governments not to ignore a spate of suspected Russian assassinations in Britain and the US, exposed by BuzzFeed News last year, as part of President Vladimir Putin’s assault on the global democratic order.

The 200-plus-page report released on Wednesday warned that “a number of individuals, including vocal Putin critics, investigative journalists, and others in the Kremlin’s crosshairs, have died beyond Russia’s borders”, and that such suspected political assassinations are part of Kremlin’s efforts “to promote a climate more conducive to the Russian government’s corrupt and anti-democratic behavior” in the West.

The report refers extensively to the two-year investigation by BuzzFeed News that revealed explosive evidence connecting 14 deaths in the UK and one in the US to Russia, and called on the governments of both countries not to turn a blind eye. “It is not inconceivable that the Kremlin could use its security services in the United States as it has elsewhere,” they wrote. “The trail of mysterious deaths, all of which happened to people who possessed information that the Kremlin did not want made public, should not be ignored by Western countries on the assumption that they are safe from these extreme measures.”

The report pointed to the evidence uncovered by BuzzFeed News on the death of Perepilichnyy and also highlighed the case of Mikhail Lesin, the founder of the news and propaganda network RT, who was found bludgeoned to death in his hotel room in Washington, DC, in 2015. The US government ruled the death an accident, but BuzzFeed News revealed last year that Lesin’s battered body was discovered on the eve of a planned meeting with the US Justice Department and that multiple intelligence and law enforcement officials said he was bludgeoned to death. BuzzFeed News also exposed holes in the police investigation.

Heidi Blake is the UK investigations editor for BuzzFeed News and is based in London.